1. Field of the Invention
The present invention generally relates to an iris control apparatus for a video camera and is directed more particularly to an iris control apparatus for use with a video camera having manual and automatic modes for manually and automatically controlling the iris, respectively.
2. Description of the Prior Art
A typical example of conventional iris control apparatus for a video camera will be explained hereinafter with reference to FIG. 1.
In FIG. 1, a reference numeral 1 designates a lens system and 2 an iris. The iris 2 is driven by a motor 3 which is driven by an iris drive circuit 4. A camera head circuit 5 is provided which has a charge coupled device (hereinafter abbreviated as a CCD) for delivering image pickup signals of three colors (blue, green and red) sample and hold circuits for sampling and holding the three color image pickup signals respectively, and pre-process circuits for performing gain adjustments, flare correction and pre-knee correction, etc., with respect to the three color image pickup signals and so on, respectively.
The red, green and blue chrominance signals R, G and B from the camera head circuit 5 are supplied to a signal processing circuit 6. The signal processing circuit 6 has a matrix circuit for generating red and blue color difference signals R - Y and B - Y and a luminance signal Y on the basis of the red, green and blue signals R, G and B, a gamma correction circuit, a knee correction circuit and an automatic white and black level balance adjusting circuit, etc.
An encoder 7 of National Television System Committee's system (hereinafter abbreviated as a NTSC system) encodes the red and blue color difference signals R - Y and B - Y and the luminance signal Y to generate a composite color video signal of the NTSC system and delivers the same to an output terminal T.
A non-additive mixer (hereinafter abbreviated as NAM) 8 delivers the signal having a maximum level among the red, green and blue signals R, G, and B supplied thereto from the signal processing circuit 6 (or the camera head circuit 5). The chrominance signal with the maximum level from the NAM 8 is supplied through an amplifier 9 to a mean value detecting circuit 10 and a peak value detecting circuit 11. The detected output signals from the circuits 10 and 11 are mixed with a proper mixing ratio by a potentiometer 12 and then the mixed value (level of the weighted sum of the mean value and the peak value) from the potentiometer 12 is supplied to a comparing circuit 13 so as to be compared with a reference level E supplied thereto from a reference power source 20. The comparing circuit 13 delivers an iris control signal in an automatic mode with such a level as to make the mixed level equal to the reference level E that is, it drives the mixed level to the reference level E. The iris control signal is supplied to the iris control circuit 4 through a manual/automatic mode change-over switch 17.
A manual adjuster 21 constituted by a potentiometer connected between a power supply source +B and ground generates an iris control signal in a manual mode, which is supplied through the change-over switch 17 to the iris driving circuit 4. One of the iris control signals from the manual adjuster 21 and the comparing circuit 13 is selected by the change-over switch 17 and supplied to the iris driving circuit 4 to thereby control an aperture of the iris 2 manually and automatically, respectively.
In the conventional iris control apparatus for a video camera described above, the automatic iris control signals from the comparing circuit 13 and the manual iris control signal from the manual adjuster 21 are independently set, that is, without any relation therebetween. Thus, if there is a level difference between these automatic and manual iris signals before and after changing over the switch 17, there arises such a drawback that the aperture of the iris 2 changes abruptly upon changing over the switch 17.